This protocol is based on the hypothesis that human fetuses grafted with allogenic bone marrow stem cells before 14 weeks of gestation will develop stable and long lasting chimerism. The basis for the hypothesis is the observation that T cells do not peripheralize from the human fetal thymus until 14 weeks of gestation, together with the finding that animals grafted with stem cells before they have t cells develop specific immunologic tolerance. Organ damage causes severe morbidity and mortality in several genetically determined metabolic storage diseases and bone marrow transplantation has been attempted to prevent this damage. Experimental models in laboratory animals, plus limited experience in man, suggest that organ damage can be slowed or prevented by transplantation of bone marrow. The mechanism for protection is most likely the colonization of the recipient with a population of marrow derived, metabolically normal, cells of the mononuclear/macrophage lineage, including tissue macrophages. Infants with storage diseases generally have normal immune responses so graft refection due to specific immunity is a major barrier to transplantation. We propose that transplantation of fetuses, prior to their developing mature specific immunity, will result in donor-specific tolerance and will facilitate postnatal transplants intended to boost the levels of donor chimerism. Fetuses with thalassemia or lysosomal storage disorders will be identified by the genetics/metabolism service using specific PCR, biochemical/enzyme analysis or linkage applied to chorionic villous biopsies obtained at 8-12 weeks gestation. Families with affected fetuses will be offered entry subject to specific criteria and admitted to the GCRC at UCHSC by the 14th week of gestation. Marrow will be drawn from an appropriate donor, processed to deplete T cells and enrich for stem cells and then injected into the peritoneal cavity of affected fetuses by 14 weeks gestation. Pregnancies will be monitored every 14 days through week 26 by echo. A fetal blood sample (FBS) will be obtained at 20 weeks and tested for chimerism and alloantigen-specific tolerance. Fetuses will be supported as required and delivered at term. Newborns will be re-tested for chimerism in the erythroid and white cell series, along with their alloreactivity to donor-specific antigens. These studies will determine whether fetal graft recipients have acquired specific immune tolerance.